A Major Hubble Survey of the Kuiper Belt

byPaul GilsteronApril 9, 2019

You’ll recall that well before New Horizons completed its primary mission at Pluto/Charon, the search was on for a Kuiper Belt Object that could serve as its next destination. Eventually we found Ultima Thule (2014 MU-69), from which priceless data were gathered at the beginning of January. Finding the target wasn’t easy given the distances involved and the small size of the relevant objects, which is why the Hubble Space Telescope was brought into the search.

The starfield in Sagittarius is crowded as we look toward galactic center, but despite the efforts of both the 8.2-meter Subaru telescope in Hawaii and the 6.5-meter Magellan telescopes in Chile, no KBOs among those found were within range of New Horizons. It was Hubble that made the difference, and Hubble which will presumably return a second target, if indeed the New Horizons team is granted an extended mission that can reach it. It’s worth noting, too, that it was Hubble that helped New Horizons in its discovery of Pluto’s smaller four moons, while also performing searches of the system for any dust rings that could harm the mission.

KBOs have never been heated by the Sun, so they provide the most pristine sample available of the earliest days of system formation. What we’ve learned about the Kuiper Belt so far is that there are a large number of binary objects within it, and as Southwest Research Institute scientist Alex Parker notes, many of these consist of two objects of similar mass. Parker will lead a new survey on the Kuiper Belt awarded to SwRI by the Space Telescope Science Institute (STScI), one that will put the emphasis on characterizing these binary populations.

“These binary systems are powerful tracers of the processes that built the planets,” says Parker. “We will use Hubble to test the theory that many planetesimals formed as binary systems from the get-go, and that today’s Kuiper Belt binaries did not come from mergers of initially solitary objects.”

Image: The SwRI-led Origins Legacy Survey will search for Kuiper Belt objects such as those shown in this artist’s illustration of a widely separated binary. Credit: Courtesy of Southwest Research Institute and Alex H. Parker.

Called the Solar System Origins Legacy Survey (SSOLS), the project represents the largest Hubble Solar System program ever, with 206 Hubble orbits around Earth allocated to it. SSOLS is conceived as a way to examine the primordial planetesimal disk with new and archival data. At stake are differing models of planetesimal formation, which predict different size and color distributions for solitary KBOs and their binary cousins.

The process of accretion would imply objects formed in isolation, later merging into binaries. In this case, the objects in binary systems would likely show dissimilar colors and a different size distribution than single KBOs. But if a process of rapid collapse was at work, producing some binary systems and some single KBOs quickly, then the expectation is for both objects in a binary system to have a similar surface color and a size distribution similar to what we find among solitary objects. At present, Hubble is the only instrument that can measure the binary occurrence rate in the Kuiper Belt, as well as the binary separation and color distribution.

SSOLS will characterize the binary and color properties of 221 KBOs, drawing on objects observed by the two largest Kuiper Belt surveys yet conducted, the Outer Solar System Origins Survey (OSSOS) and Canada-France Ecliptic Plane Survey (CFEPS). This earlier work becomes the framework within which the binary characterization of KBOs can proceed. For more, see the SSOLS website at https://www.ssols.space/, and ponder the need for the next outer system spacecraft that can take us into the realm New Horizons continues to explore.

“The process of accretion would imply objects formed in isolation, later merging into binaries.” Why is it automatically assumed that objects will coalesce into binaries ? It would seem the spacing of these objects would preclude against that happening.

I’m not certain they know that Ultima Thule is actually a binary. It may or may not be, but my point is that I think it would be seemingly more likely that bodies would not collide in that relatively vast expanse. That’s just my opinion.

If binaries are common in the Kuiper belt, they are presumably so in the Oort too. We should, therefore, expect that comets should also commonly be binary objects, like 67P/Churyumov–Gerasimenko originating from the Kuiper belt. This suggests to me that observations of fresh comets while in the outer system could also shed light on this question of formation.

One thing that we tend to forget is that Hubble Space Telescope is based on technology that is over 30 years old. Although it has been updated numerous times, the last mission to it was ten years ago! What plans are there to reservice it in the near future? This telescope has had more publicity then any telescope in existence and is in earth orbit only 370 miles from you or I. Based on solar activity and atmospheric drag, or lack thereof, a natural atmospheric reentry for Hubble will occur between 2028 and 2040. In June 2016, NASA extended the service contract for Hubble until June 2021.
A servicing mission with the new manned spacecrafts and some hardware launched to upgrade it, would make for a good demonstration of the the USA abilities in space!

white elephant. An unwanted or financially burdensome possession. Unfortunately that is what the JWST appears to be. All the news about it continues to be bad with cost overruns and continuing delays before its launch. I would definitely be thinking about another maintenance visit to HST if I were NASA. The new committed launch date is March 2021 but who says that will happen? A new projected total cost is 9.66 billion (although the agency appears to be sticking to the 8.8 billion dollar price tag as of now)! Astounding and horrifying.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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